- No use inhibitors or inducers of the cytochrome p450 system CYP3A4 enzyme or other medications such as aprepitant, ketoconazole, itraconazole, quinidine, digoxin, cyclosporine, ritonavir, grapefruit products, St. John's Wort, rifampin (rifampicin), carbamazepine, phenytoin, dexamethasone, and phenobarbital.